Process for manufacturing electrodes for gaseous electric discharge apparatus



y 1957 P. RASEMONT 2,798,828

PROCESS FOR MANUFACTURING ELECTRODES FOR GASEOUS ELECTRIC DISCHARGE "APPARATUS Filed March 10, 1953 'j II/VVEIVTORJHUI Unite PROCESS FOR MANUFACTURING ELECTRODES FOR GASEOUS ELECTRIC DISCHARGE APPA- RATUS Pierre Rasemont, Rosny-sous-Bois, France, assignor to Societe Anonyme pour les Applications de IElectricite et des Gaz Rares-Etablissements Claude Paz & Silva, a French Cy.

Application March 10, 1953, Serial No. 341,449

Claims priority, application France March 26, 1952 2 Claims. (c1. 148-6) stance, and by subjecting the whole to a suitable thermal treatment.

The heating of the assembly of the filament and mixture, however, leads to the starting, throughout the mass, of a very violent reaction, which causes the spattering of a substantial proportion of the activating mixture and an important deformation of the support thereof. When the thermal treatment is effected by means of the Joule effect, by connecting the ends of the filamentary support with a suitable current source, the fierceness of the reaction which starts over the entire length of the support and in the bulk of the mixture, causes not only a spattering of emissive materials but also deformations of the coil formed by the support, which show in a tightening of the coil turns at some points and a stretching at other points. There results very large irregularities of the electrodes thus manufactured (irregularities in the amount and distribution of the emissive materials, in the heating of the coil turns in operation) and consequently in the qualities of the lamps made with such electrodes.

One object of the invention is to avoid having a fierce reaction when a mixture of alkaline-earth dioxide and metal particles embedding a filamentary support of tungsten or molybdenum is heated sufiiciently for promoting a reaction in this mixture.

Another object of the invention is to provide electrodes having a long life, even when they are not pre-heated for starting the discharge, by the use of relatively large amounts of emissive materials, these electrodes, however, not having the usual drawback of electrodes with large emissive coatings, of rapidly showing stains.

A feature of the invention consists in coating at least a part of a filamentary support and at least a portion of an electrically conducting member with an activating mixture comprising at least metal particles and alkalineearth metal dioxide, said support part, said member portion and the coating mixture on both forming one and same composite body, and heating said coated member portion above the temperature at which said dioxide just reacts upon at least one of the metals constituting the metal particles and the filamentary support While heating said spport substantially under said temperature.

A further feature of the invention consists in starting the reaction locally only, by heating by one or more portions of electrically conducting member or members, for instance portions of the lead-in wires to the electrodes; said member portions are preferably placed towards the periphery of the composite body. The reaction States PatentO M ice is then much more gradual, as it spreads inside a relatively cold body and from a small number of points only, preferably located near the periphery of this body. It is relatively quiet and does not cause any important deforma tion of the spiral, nor any spattering. There is thus obtained a thick, adhering, and suitably distributed emissive coating.

The supports consisting of tungsten or molybdenum coiled filaments generally used for constituting the electrodes, do not heat up, practically, when subjected to the high frequency magnetic field generally used. for induction heating. This is due, on the one hand, to the fact that the coils do not constitute a closed circuit, and are not made of ferro-magnetic metals, and above all, to the fact that the filaments being very thin, eddy current losses therein are very low.

On the contrary, if the lead-in wires to these filamentary supports are made of term-magnetic material, as is usual, or are sufficiently bulky, they are raised to temperatures which, though moderate, are sufficiently high for starting the reaction between the barium dioxide, the tantalum and the metal of the support, when said lead-in wires are in contact with the materials to be reacted. Other small electrically conducting masses than the lead-in wires may play this part. i

According to one preferred embodiment of the method of the invention, there is used, for heating, a high frequency magnetic field which raises said conducting mass or masses to a temperature at least equal to that necessary for starting the reaction; the filamentary support for the emissive coating and its connections should then be given such an arrangement, dimensions and nature that the high frequency field used raises said support only to a temperature very substantially lower than the temperature necessary for starting the reaction.

In the method according to the invention, there does not occur, under the action of the high frequency field, far from it, a sufiicient heating of the filamentary support itself, for starting the reaction. On the other hand, sufficiently bulky electrically conducting masses, subjected to said field, for instance the lead-in wires to the support, are raised to a sufficient temperature, so that the reaction starts at their contact in the activating mixture which will form the emissive materials; the reaction then spreads from these masses.

In drawings, which illustrate embodiments of the invention:

Figure 1 is a diagrammatic assembly view, of an apparatus making it possible to apply the process of the invention.

Figures 2, 3, 4 represent modifications of the assembly comprising the filamentary support for the emissive coating, the activating mixture with which the support is provided before the reaction and the conducting mass or masses used for starting the reaction.

The mixture which with a thermal treatment will give the emissive coating, is obtained by milling barium dioxide and a metal powder, tantalum powder in the present case (size 200 mesh), mixed with acetone. Possibly acetone is added after milling in order to form a suitable pasty consistency. The degree of milling makes it possible to control the speed of propagation of the reaction.

An activating mixture can be used, for instance, containing:

Grams Barium dioxide 66 Powdered tantalum 44 Acetone The support 7, in the illustrative cases, is a coiled tungsten coil. It might, however, be of other shapes, such as a simply coiled filament, bare or surrounded with another filamentary coil, with or without touching turns, or it might consist of several parallel filaments coiled to the .same shape, :etc. It may also .be made of molybdenumor consistof .both tungsten and molybdenum filamerits.

In the case illustrated in Figure 1, the support 7 is mounted, by its lead-in wires, 4 and 5, on the glass stem 1, by means .of which latter the electrode/to be obtained will be secured in the discharge apparatus for which it is intended. The support 7 is coated with the above mixture, for instance by dipping it thereinto .and withdrawingit. The amount of mixture coating the support may.

be increased by repeating this operation .one or more times, the mixture being dried roughly between two-dippings, forinstance by-a .two minute exposure to air. Not only the support? is thus .coatedwith activating mixture, but also the lead-in wires 4.,and 5 at the. points where the support 7 is fastened to them and in the vicinity of those points; the same procedure-is followed even if one of these lead-in wires is not intended to be used later for bringingcurrent to'the electrode, but .is intended only to support the electrode.

The coating 6 is then, .possibly, .dried morethoroughly for instance by being left in air for fifteen minutes.

The stem 1, on which is mounted the support 7 coated with the layer .6 is then placed onone end of a tube 3 which is a part of a mounting 2, made of -silica, for instance. Then, preferablyi a reducing gas is passed through this mounting, for instance, .a mixture of nitrogen and hydrogen, or simply a non oxidizing gas such as nitrogen. The gas, for example, is introduced through atube 9 at the other. end of the tube 3 .andissues from the portion of the mounting 2 where the support is located, through holes 10 in the tube 3. The gas is allowed to flow during the heat treatment which will be effected later, including at least the beginning of the cooling which terminates this treatment. Good results may also be obtained by leaving some air .in the mounting, provided the air renewal is prevented, for instance by stopping the upper or lower aperture of the mounting or both, at least roughly.

A coil 8, which surrounds the mounting 2 at the height .of the support 7,-is then connected for a short While to the terminals of. an alternating current source with afrequency of the order of 2000 kilocycles. The magnetic field thus generated raises immediatelythe support 7 and the coating 6 to a temperature at which a reaction starts in the coating; the power supply to the coil 8 may then be cut off. The reaction lasts onlyfor about one second, after which the support and its coating are allowed to cool for about 10 seconds. The coating then forms a dark gray or black mass covering the surface of the filament or filaments constituting the support, filling the intervals between adjacent turns, and in the case of a double-coiled support filling the turns of the small turn coil. The filamentary support and its coating then form an electrode which it is sufficient to secure by its stem on the envelope of a discharge apparatus and toform by .a'very simple treatment for instance by a gas removal in vacuum by a Joule eifect obtained, as known, by connecting the terminals of the support to a suitable current source; this gas removal may be carried out at the same time as that of the envelope, obtained, for instance,.by heating the latter in an oven.

This electrode may be kept in the open air during a time amply sufiicient for allowing the next operations.

It is advantageous to coat it, as well as its currentleads, with a coating of alumina. This coating decreases the action, detrimental though slow, of the air constituents andofthecombustion gases which may come in contact with the electrode during the'sealing of the stem to the envelopegthe alumina coatinghas, further, a favourable effecton the electrode characteristics and it decreases the discharges which-may originate on .-.the current loads.

This coating, obtained, for instance, by spraying, may be eifected after, or better before, the above mentioned heating by means of the high frequency magnetic field.

If, during this heating, the current leads 4, 5 are connected in an electric circuit, the latter should be such that it would not allow, in the support subjected to the high frequency magnetic field but assumed to be not coated with the activating mixture, a current flow of an appreciable density.

The above method makes it possible to obtain thick emissive coatings, constituting therefore an important storage, which are however regular and adhering and therefore easy to degas by heating at the time of pumping and which can be used until their complete volatilization. For example, an electrode designed for a current of 250 to 500 ma. may comprise an emissive coating containing an amount of barium corresponding to 15 to 20 milligrams of BaO; on the other hand, on a support having the same dimensions but coated in accordance with usual methods, the coating generally corresponds only to 5 milligrams of .BaO; if the number of layers deposited in the latter .case is increased, the coatings ,are irregular, which makes it ditficult to correctly-exhaust the discharge apparatus, hence numerous'drawbacks, such .as swirling and the rapid formation of stains.

In the aboveexample, of an electrode designed for 250 to 5.00 mililamperes, the emissivelayer support 7 consists, forinstance of a coiled coil of tungsten filament 0.009 mm. in diameter, the primary coil being 0.2 mm. in diameter and the secondary .coil 0.65 mm.; this'support is heldziby two lead-in wires 4 and 5, madeof nickel,=0:6 mm. .in diameter, which, before "heating by a high frequency field, are embedded over a length of at least 2 mm. .inside the activating coating '6. If this filamentary support, mounted on its lead-in wires, butwithout any activating mixture is subjected to the same high frequency magnetic field as theone used for manufacturing the electrode, it isfound that said support barely reaches C., while the temperature of the lead-in wires reaches a value between 350 and 450 C.

It maybe convenient to constitute the lead-in wires of the electrodes in such a manner that their heating under the action of the high frequency field be insufficient to start a reaction; in such a case, there will be arranged, either in the coil constituting the support for the emissive materials, as shown in Figure 2 orin its ,activating coating, or in contact therewith, as shown in Figure 3 a metal mass 12 or 13, of small dimensions, but such, however that it heats up sufficiently, under the action of the high'frequency for causing a local starting of the reaction. The same procedure will .be followed if the ends of the lead-in wires-are not inside the mixture with which the support for the emissive materials has beenprovided.

The above described method may be modified-in numerous ways within the scope of the present invention. In particular tantalum may be used in grains of different sizes, or even as an impalpable powder and tantalum may be replaced, partly at least, by another metal such as molybdenum, tungsten, zirconium, columbium; theemissive coating support may be provided, for instance, with the following mixture:

Barium dioxide grams 40 Tantalum powder do 5 Zirconium powder do 3 Acetone cm. :50

.Thcactivating mixture may contain inert or poorly reactive materials, for instance with a view to slowing down thereaction described or modifying the properties of the electrode. Various forms of mounting may be devised; the coated support may be subjected to the magnetic field without having been, previously, secured to its stem, etc.

The support itself may offer other shapes than spirals or helices, simple or multiple, provided it constitutes a.

circuit in which the magnetic field can induce only a current of low or zero density. The lead-in wires may have any usual shape for carrying the filamentary support; they may be straight, bent, or have, as shown in Figure 4 at 14 and 15, curved ends entering the ends of the spiral along which the filament support is wound, etc.

There are provided by the process of the invention electrodes which have a long life and do not rapidly cause stains on the envelope of the discharge apparatus in which they are mounted. This process gives these results regularly and may be carried out by machines individually or adapted to mass production.

The portion 13 of stout wire which is the heating metal mass in the assembly represented on Figure 3 may be heated either by a high frequency magnetic field, or by Joule effect, by connecting the ends of this wire to a suitable current source.

I claim:

1. The process for manufacturing an electrode for gaseous electric discharge apparatus, said electrode having an emissive coating and a filamentary support therefor, said support being attached to at least one current lead-in wire and consisting of at least one coiled wire of a diameter substantially smaller than that of the lead-in wire, said coiled wire being made of a metal selected from the group consisting of tungsten and molybdenum, which process comprises coating said filamentary support and a portion of the leadin wire with an activating mixture comprising alkaline-earth metal dioxide with particles of at least one metal selected from the group consisting of tantalum, zirconium, columbiurn, molybdenum and tungsten so as to cause said support, said portion of the lead-in wire and the coating mixture therein into one composite body, placing said composite body in a high frequency magnetic field of a strength and frequency sufliciently great for heating said lead-in wire portion above a temperature at which said alkaline-earth metal dioxide Will just react upon at least one of the metals constituting the metal particles and the filamentary support, and being sufiiciently small for heating said support very substantially under said temperature to render the reaction between barium dioxide and the metal particles much less violent, whereby a thick emissive coating is produced on the electrode without deformation of the support and irregularities of the electrode.

2. The process as set forth in claim 1, including the step of providing the lead-in Wire with a metal mass of small dimensions in contact only with a small proportion of the activating mixture to be heated sufficiently by the high frequency magnetic field for causing a local starting of the reaction.

References Cited in the file of this patent UNITED STATES PATENTS 1,579,009 Langmuir Mar. 30, 1926 1,591,717 Marden July 6, 1926 1,813,320 S-chaetfer et al. July 7, 1931 1,850,809 Robinson Mar. 22, 1932 1,946,498 Noble Feb. 13, 1934 1,977,318 McCullough Oct. 16, 1934 2,023,707 Spanner et al. Dec. 10, 1935 2,106,754 Lederer Feb. 1, 1938 2,362,510 Stutsman Nov. 14, 1944 2,657,325 Homer et a1. Oct. 27, 1953 

1.THE PROCESS FOR MANUFACTURING AN ELECTRODE FOR GASEOUS ELECTRIC DISCHARGE APPARATUS, SAID ELECTRODE HAVING AN EMISSIVE COATING AND A FILAMENTARY SUPPORT THEREOF, SAID SUPPORT BEING ATTACHED TO AT LEAST ONE CURRENT LEAD-IN WIRE AND CONSISTING OF AT LEAST ONE COILED WIRE OF A DIAMETER SUBSTANTIALLY SMALLER THAN THAT OF THE LEAD-IN WIRE, SAID COLIED WIRE BEING MADE OF A METAL SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN AND MOLYBDENUM, WHICH PROCESS COMPRISES COATING SAID FILAMENTARY SUPPORT AND A PORTION OF THE LEAD-IN WIRE WITH AN ACTIVATING MIXTURE COMPRISING ALKALINE-EARTH METAL DIOXIDE WITH PARTICLES OF AT LEAST ONE MWTAL SELECTED FROM THE GROUP CONSISTING OF TANTALUM, ZIRCONIUM, COLUMNIUM, MOLYBDENUM AND TUNGSTEN SO AS TO CAUSE SAID SUPPORT, SAID PORTION OF THE LEAD-IN WIRE AND THE COATING MIXTURE THEREIN INTO ONE COMPOSITE BODY, PLACING SAID COMPOSIT BODY IN A HIGH FREQUENCY MAGNETIC FIELD OF A STRENGHT AND REFQUENCY SUFFICIENTLY GREAT FOR HEATING SAID LEAD-IN WIRE PORTION ABOVE A TEMPERATURE AT WHICH SAID ALKALINE-EARTH METAL DIOXIDE WILL JUST REACT UPON AT LEAST ONE OF THE METALS CONSTITUTING THE METAL PARTICLES AND THE FILAMENTARY SUPPORT, AND BEING SUFFICIENT SMALL FOR HEATING SAID SUPPORT VERY SUBSTANTIALLY UNDER SAID TEMPERATURE TO RENDER THE REACTION BETWEEN BARIUM DIOXIDE AND THE METAL PARTICLES MUCH LESS VOILENT, WHEREBY A THICK EMMISSSIVE COATING IS PRODUCED ON THE ELECTRODE WITHOUT DEFORMATION OF THE SUPPORT AND IRREGULARITIES OF THE ELECTRODE. 